IThis essay explores Benjamin Rush's ideas about physiology in an effort to revise current understandings of Rush's medico-political model and shed new light on conversations about circulation and sympathy in the early republic. Rush's non-hierarchical model of circulation broke with European medicine. For Rush, circulation was the key to corporeal and national health. Circulation needed to remain unfettered for individuals to realize republican promise and for the body to be properly invigorated—but free flow was problematic when it allowed information, goods, and bodies to flow unchecked. Sympathy was the secondary, essential mechanism that controlled this movement. Whereas circulation importantly opened both body and country to external stimuli, sympathy—physiological, social, political—managed responses to those stimuli, directing them along salubrious routes that were both natural and teachable. Rush himself worked tirelessly to mold these sympathies through rhetoric. This physiology and Rush's rhetorical medicine challenge the common understanding of Rush's "republican machines"; American bodies were, rather, dynamic living systems that could, through the cultivation of proper sympathies, become virtuous citizens. This essay extends current work on circulation by reconnecting it to physiology and suggesting that physiology's dynamism—rather than static "anatomy"—ought to inform discussions of the young nation. Rush knew bodies and nations were "tremendous oscillatory mass[es] of matter." America would maintain national health not by restricting circulation but by influencing citizens' reactions to free-flow systems that were only somewhat predictable and always dynamic. This physiology provides a new model for thinking about early American circulation and sympathy.